Magnetic resonance imaging (MRI) is a common modality for imaging joints and other parts of the body due to its excellent definition of ligaments, cartilage, bone, muscle, fat and superior soft tissue contrast. MR techniques are utilized in multiple applications to determine whether structural defects are present in a target being imaged.
When a substance, such as human tissue, is subjected to a uniform magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field that is in the x-y plane, and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization,” may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment. A signal is emitted by the excited spins after the excitation signal is terminated, and this signal may be received and processed to form an image.
In order to produce the magnetic fields used in MRI applications, Radio Frequency (RF) coils are utilized to generate the required magnetic fields to accomplish the above functionality. In general, RF coils for MRI attempt to produce a uniform field in the target volume which is uniform in both amplitude and phase because such properties provide for an image with reduced artifacts. Generally in RF coil design, providing linear phase variations along the imaging volume is not a primary concern. This is particularly true with respect to microstrip RF coil designs which do not utilize such a coil to produce a linear phase variation.
One previous attempt of designing an RF coil which produces linear phase variation is a twisted birdcage coil design. In this design, an existing RF coil type (birdcage coil) is taken and twisted, causes the phase in the x-y plane (the useful plane for imaging purposes) to vary as a function of position in the z direction (along the axis of the coil). However, this design has multiple drawbacks. For example, while the actual twisting of the twisted birdcage coil allows for the linear phase variation properties in the x-y plane, the twist in the coil increases the field in the z-direction. As a result, a large field in the z-direction is created when achieving any significant linear phase variation properties. A large field in the z-direction is undesirable because it deposits additional power within the target object being imaged, while not providing meaningful assistance with the actual imaging. When high power deposition is present, the entire imaging process must slow down in order to avoid excessive heating of the target object. The twisted birdcage coil also relies on two end rings at each end of the coil. These rings have very high electric fields associated with them which will cause additional power deposition to take place in the target object in an area proximate to the end rings.